1. Field of the Invention
The present invention relates to an optical system and, more particularly, to an optical system suitably applicable to use in image viewing apparatuses, such as viewfinders of cameras, so-called head-mounted displays, or the like, and in image pickup apparatuses, such as camera or the like.
2. Related Background Art
For the head-mounted display etc., various optical systems have been devised heretofore for making compact the overall apparatus.
For example, Japanese Laid-open Patent Application No. 58-78116 discloses an optical apparatus for a viewer to view an object image on a primary image plane, obtained by a photographing optical system, through an optical element of a prism shape having a concave, spherical reflecting surface inclined with respect to the eye of the viewer.
Since in this conventional example the reflecting surface is an inclined, spherical surface, it is difficult to correct optical aberrations such as astigmatism etc. occurring thereat into good order. The above Japanese application describes the necessity for addition of a new lens system for improving the aberrations.
Japanese Laid-open Patent Application No. 5-303055 (the Japanese counterpart of U.S. Pat. No. 5,594,588) discloses an apparatus additionally including such a lens system (relay lens system). The addition of such a relay lens system, however, involves the disadvantages of increasing the total length of the optical system and degrading the compactification of the optical system.
In order to improve the compactification and the aberrations of the optical system, Japanese Laid- open Patent Applications No. 5-303054, No. 5-303056 (the Japanese counterpart of USP No. 5,594,588), and No. 5-323229 disclose optical systems in which the reflecting surface is one reflecting surface comprised of a general, rotationally asymmetric surface having higher-order aspherical terms, a paraboloid of revolution or an ellipsoid of revolution according to a conical function defined by a conical coefficient, or a toric, aspherical surface (or an anamorphic, aspherical surface) represented by different aspherical functions on the orthogonal coordinate axes respectively and is inclined with respect to the optical axis of the viewer's eye.
The optical systems of the structures having the reflecting surface of such one aspherical surface, however, had the drawback of incapability of sufficiently correcting all of deformation of image (distortion), curvature of field, and difference between foci in orthogonal directions (astigmatism).
In the case of the so-called decentering optical systems with the reflecting surface thus inclined, optical performance will vary greatly if focus adjustment (diopter adjustment) of the optical system is effected by moving the optical system. Therefore, a desired method for the focus adjustment is one for adjusting the focus by moving the focal plane of the optical system (an image display element or an image pickup element). For that, it is desirable to make the optical system telecentric with respect to the focal plane.
Let us check the conventional examples described above from these viewpoints. In Japanese Laid-open Patent Application No. 5-303054 correction is made for astigmatism and distortion but curvature of field still remains without being corrected.
In Japanese Laid-open Patent Application No. 5-323229, correction is made for curvature of field, astigmatism, and distortion, but angles of rays are inclined largely with respect to the focal plane. If a display element or an image pickup element such as a liquid crystal panel, a CCD, or the like is placed on the focal plane there will arise a drawback of considerable degradation of performance due to angular dependence of characteristics of these elements.
In the optical systems suggested in these applications, though distortion is corrected, deformation of the image is of a level in which is can be discriminated at a glance and thus the correction is not sufficient.
As described above, the structures with the reflecting surface of one aspherical surface had the drawback of incapability of sufficiently correcting all the aberrations.
Further, Japanese Laid-open Patent Application No. 3-180810 discloses an optical system in which distortion of the image plane is corrected by an aspherical surface having cross terms (xy etc.) of the orthogonal coordinate system (x, y, z) while the visual point of the viewer is moved away from the optical system to increase FNo to the optical system and increase the depth of focus of the optical system.
This optical system, however, is based on the method for viewing the image from the distant visual point and, therefore, cannot be constructed in a wide angle or large screen configuration. Although the optical system is constructed in the telephoto type by combining a convex mirror with a concave mirror, the overall size is large. Therefore, the optical system had the drawback of incapability of being constructed in compact structure.